It used to be available in supermarkets with a large Jewish clientele (e.g. Côte St-Luc). Haven't checked recently, though. A thread from 2005 has a few other ideas: http://chowhound.chow.com/topics/97951

Dr Browns and Stewart's are not the same it is a taste from childhood that is the hook and the density of bubbles. I still can't find the cases of the stuff. The butcher shop that is also a sausage counter on St Urbain above Bernard has them but he only sells them by the unit.Please help !

The Asian run grocery on Sherbrooke in Westmount (forget the name of the store) sells them, both regular and diet in cans. They also have a large selection of other canned and bottled root beers including Dr. Brown's, Stewart's and Boylans. It's the one right near Victoria...

maybe not in cases but I am 100% certain that I've bought them in cans from the fridge in the back last summer. I remember because I filled an entire shopping basket with them. I haven't been in months but that much I know for sure.